1. Field of the Invention
This invention relates to a method for the production of a magnetic recording medium to be used for recording and reproducing images, voices, data, etc.
2. Description of the Prior Art
Heretofore, in the manufacture of a magnetic recording medium provided on a nonmagnetic support thereof with a magnetic layer containing a magnetic powder, the practice of forming a backcoat layer on the surface of the nonmagnetic support opposite the surface of the magnetic layer for the purpose of improving the running property of the medium itself has been in vogue. While the backcoat layer of this sort indeed contributes to the improvement of the running property, however, the fact itself of providing the backcoat layer for the magnetic recording medium necessitates a work of applying a paint to the support and drying the applied layer of the paint besides the work performed on the magnetic layer and consequently complicates the process of manufacture of the recording medium and directly bears on the cost of manufacture. Thus, the adoption of this practice has been limited to magnetic recording media of high quality. More often than not, the backcoat layer in popular use to date has incorporated therein a pigment of a large particle diameter for the principal purpose of improving the running property of the magnetic recording medium.
In recent years, the demand made by the magnetic recording medium for higher recording density has been urging the magnetic layer toward a smaller thickness, a greater degree of packing, and higher durability. Thus, targets of high level have come to be set for the magnetic layer.
For the purpose of realizing such a magnetic layer as is aimed at, it has been proposed to form the magnetic layer in a two-layer structure or provide the magnetic layer with an undercoat. Concerning the manufacture of the magnetic layer of such a modified structure as mentioned above, a study is now under way on the feasibility of adopting the wet-on-dry coating method which comprises first drying the lower layer and then forming the magnetic layer as an upper layer and the wet-on-wet method which comprises forming the layer on the lower side and the magnetic layer on the upper side simultaneously while they are both in a wet state. As the prior art associated with these methods, JP-A-53-92,110 discloses a method of production which resorts to application of a magnetic field to the two-layer film and JP-A-62-212,933 discloses a method for the production of a magnetic recording medium which comprises simultaneously applying at least two magnetic layers in a superposed manner and applying a magnetic field on the two layers before they are dried a thereby effecting orientation of the magnetic powder in the magnetic layers. Then, JP-B-05-59,490 proposes a method for the production of a magnetic recording medium which comprises forming a magnetic layer and a nonmagnetic undercoat layer simultaneously by the superposing coating technique and applying a magnetic field on the two layers before they are dried thereby orienting the magnetic powder in the magnetic layer. Further, JP-A-05-81,667 discloses a method for the production of a magnetic recording medium which comprises applying in a superposed manner a paint for first application and a paint for last application, both having magnetic particles of pure iron or a metal alloy dispersed therein, by the use of a first coating nozzle and a last coating nozzle so as to have the paints to be deposited either separately or integrally in a wet state and thereafter performing a treatment for smoothing the surface on the applied layers. Then, JP-A-07-176,047 proposes a method for the production of a magnetic recording medium which comprises a step of either simultaneously applying two magnetic layers one each to the opposite surfaces of a support or applying a magnetic layer to one surface of the support and then applying another magnetic layer to the remaining surface before the first magnetic layer dries. This method is aimed at equalizing the properties of the magnetic layers on the two sides of the support.
In the application of a magnetic layer to a substrate, when the magnetic layer is formed in a decreased thickness, the freshly applied magnetic layer dries quickly (the solvent therein readily vaporizes) because of the small thickness of the layer and, in spite of the adoption of any of the coating methods proposed as described above, the subsequent treatment for the orientation of a magnetic field is liable to fail to improve so much the degree of orientation of the magnetic powder in the applied coat as is expected. When the freshly applied magnetic layer dries at an extremely high speed, the paths through which the solvent vaporizes from the layer leave behind a porous surface to the layer, with the possible discouraging result that the intended impartation of a higher degree of packing and enhanced durability to the magnetic layer will not be fulfilled. Further, the task of efficiently forming the backcoat layer remains yet to be fulfilled. Absolutely no technical disclosure concerning the efficient formation of the backcoat layer is found anywhere in the patent publications cited above.
This invention has been created in the light of the true state of prior art mentioned above. An object of this invention is to allow formation of a magnetic layer excelling in the degree of orientation in spite of a decrease in thickness, ensures impartation of a high degree of packing and enhanced durability to the magnetic film of a small thickness, and permits efficient formation of a backcoat layer which used to impose a heavy load on the conventional process of manufacture. Another object of this invention is to exalt the surface quality of the magnetic layer by improving the operational efficiency of the calendering step and consequently decrease the loss of the applied layer of the paint which used to occur during the course of the calendering step in the conventional separate line.